Time Conversion Guide: Seconds, Minutes, Hours, Days, and More

10 min read•June 1, 2026•Reviewed: June 1, 2026

✓Reviewed by Apex Conversion Editorial Team · Last reviewed June 1, 2026

Time is the one measurement system used identically across every country — there are no metric vs. imperial time debates. Yet time conversion is still surprisingly tricky because the units do not follow a decimal pattern: 60 seconds per minute, 60 minutes per hour, 24 hours per day, 7 days per week, and irregular month lengths that make month-to-day conversions approximate.

This guide covers all common time units — from nanoseconds in computing to years in finance — with exact conversion ratios, worked examples, and the precise definitions used in programming and scientific contexts.

What the Units Mean

The second (s) is the SI base unit of time, defined since 1967 as 9,192,631,770 periods of the radiation from a cesium-133 atom's hyperfine ground-state transition. This atomic definition makes the second extraordinarily precise — modern atomic clocks lose less than 1 second every 300 million years.

Larger units: 1 minute = 60 seconds (exact). 1 hour = 60 minutes = 3,600 seconds (exact). 1 day = 24 hours = 86,400 seconds. 1 week = 7 days = 604,800 seconds. Months and years are more complex: calendar months have 28–31 days; a Julian year = 365.25 days = 31,557,600 seconds (used in scientific contexts for uniform calculations).

Key Conversion Relationships

━━━ EXACT RATIOS ━━━
  1 min  = 60 s
  1 hr   = 60 min   = 3,600 s
  1 day  = 24 hr    = 1,440 min   = 86,400 s
  1 wk   = 7 days   = 168 hr      = 604,800 s

━━━ APPROXIMATE (average month / year) ━━━
  1 Julian month = 365.25 ÷ 12 = 30.4375 days = 2,629,800 s
  1 Julian year  = 365.25 days  = 8,766 hr    = 31,557,600 s
  1 Gregorian year (common yr) = 365 days
  1 Gregorian year (leap yr)   = 366 days

━━━ COMPUTING ━━━
  1 ms  = 0.001 s   1 μs = 0.000001 s   1 ns = 0.000000001 s
  Unix epoch: seconds since 1970-01-01 00:00:00 UTC

Real-World Examples

Project management: a 90-day deadline = 90 ÷ 7 = 12.86 weeks = about 12 weeks and 6 days. A 6-month project = 6 × 30.4375 = 182.6 days ≈ 183 days = 26.1 weeks. A 2-year contract = 2 × 365.25 = 730.5 days ≈ 730 days (or 731 if a leap year falls within it).

Finance and investing: daily interest rates are calculated as annual rate ÷ 365. Monthly compounding uses ÷ 12. A 5% annual rate is approximately 5/365 = 0.01370% per day or 5/12 = 0.4167% per month. For exact daily compounding, use the actual day count between dates.

Computing and technology: a web page loading in 250 ms = 0.25 seconds. A CPU clock running at 3.6 GHz performs 3.6 × 10⁹ clock cycles per second, or one cycle every 0.278 nanoseconds. Video at 30 fps means each frame displays for 33.33 ms.

Common Mistakes to Avoid

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Assuming all months are 30 days: months range from 28 to 31 days. Use 30.4375 days as the Julian average for planning calculations, but always use actual calendar dates for contracts and deadlines.
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Forgetting leap years: a standard year has 365 days = 8,760 hours; a leap year has 366 days = 8,784 hours. Age calculations, anniversary dates, and multi-year interest calculations require leap year awareness.
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Confusing 'per hour' with 'per minute' in rate calculations: a speed of 60 mph means 60 miles every 60 minutes = 1 mile per minute. At 90 km/h, you travel 1.5 km per minute = 25 m per second.
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Using months in computing: most programming languages store time in seconds since an epoch. Converting months to seconds requires knowing the specific calendar months involved. Use ISO 8601 duration (P1M) for month-based intervals, not a fixed second count.

Frequently Asked Questions

How many seconds is 1 hour?

1 hour = 60 minutes × 60 seconds = 3,600 seconds exactly.

How many hours is 1 day?

1 day = 24 hours exactly. A Julian day in astronomy is exactly 86,400 SI seconds. Solar days vary slightly due to Earth's elliptical orbit; the difference is less than 30 seconds over any given day.

How many days is 1 year?

A common (non-leap) year = 365 days. A leap year = 366 days. A Julian year = 365.25 days (exact, for scientific use). A Gregorian year averages 365.2425 days accounting for the leap-year rule (every 4 years except centuries not divisible by 400).

How many seconds in a year?

Using Julian year: 365.25 × 86,400 = 31,557,600 seconds. Using a common year: 365 × 86,400 = 31,536,000 seconds. The difference is 86,400 seconds (exactly one day — the extra day per 4-year cycle).

How many weeks is 90 days?

90 ÷ 7 = 12.857 weeks = 12 weeks and 6 days. A 90-day period (approximately 3 months) contains 12 complete weeks plus 6 remaining days.

How do I convert minutes to hours and minutes?

Divide total minutes by 60. The quotient is hours; the remainder is minutes. Example: 150 minutes ÷ 60 = 2 remainder 30 → 2 hours 30 minutes (2:30). For decimal hours: 150 ÷ 60 = 2.5 hours.

How many milliseconds is 1 second?

1,000 milliseconds (ms) = 1 second exactly. 1 microsecond (μs) = 0.001 ms = 0.000001 s. 1 nanosecond (ns) = 0.001 μs = 10⁻⁹ s. These units appear in computing, telecommunications, and physics.

How many months is 2 years?

2 years × 12 months/year = 24 months. If the question involves specific calendar months, count them directly — February lengths and leap years matter for exact day counts.

What is Unix time?

Unix time (also called Epoch time or POSIX time) is the number of seconds elapsed since 00:00:00 UTC on January 1, 1970. As of mid-2026, Unix time is approximately 1,748,800,000 seconds. It is the standard for timestamps in programming, databases, and APIs.

How many days between two dates?

Subtract the earlier date from the later date. In most programming languages: (date2 − date1) gives a timedelta object; call .days for the result. In Excel: =DATEDIF(start,end,'D') or simply =end−start and format as a number. Remember to account for whether the end date is inclusive or exclusive.

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